The invention relates to a method for receiving signals characteristic of cardiac activities in the atrium and/or ventricle of a heart and for evaluating the signals to obtain a control signal for a heart pacemaker and/or defibrillator. The invention also relates to a device for performing the method.
Implantable heart pacemakers and defibrillators have an input stage connected to an intracardial electrode for receiving and amplifying intracardially picked-up cardiac activity potentials and an evaluation unit for evaluating them and deriving control signals for operating the pacemaker or defibrillator. If signals from the heart chamber (ventricle) as well as the atrial area (atrium) are required for the control, then one electrode is provided for each, and the input stages (if necessary also components of the evaluation unit) generally form separate channels, for which the processing characteristics (sensitivity or detection threshold, filtering and amplification parameters) can be adjusted separately.
With automatic defibrillators and double-function pacemakers, which function as defibrillators if necessary, the initial stages must also detect and preprocess as signals the cardiac action potentials or signals that appear with normal cardiac action (sinus rhythm), such as occur with the various arrhythmic conditions of the heart.
The signal amplitudes for the intracardially picked-up signals, which characterize the different heart rhythm conditions, differ considerably from each other, as can be seen in FIG. 2. Graph I here illustrates a typical sinus rhythm with normal heart function, Graph II the electrogram for a ventricular tachycardia and Graph III that of a heart chamber fibrillation (ventricular fibrillation). A suitable detection threshold TI, TII, TIII is respectively marked here with a dashed line (Graph I), a dash-dot line (Graph II) or a double dash-dot line (Graph III).
An embodiment of the initial stage of a pacemaker with an automatic gain control (AGC) is known--for example from EP 0 349 130 A1. It is designed to improve the signal/noise ratio when operating jointly with a band-pass filtering.
Furthermore, it is known from U.S. Pat. No. 4,184,493 A1 to provide an automatic gain control with an automatic, implantable defibrillator. Together with a high-pass filtering, this causes a far-reaching suppression of S and T components of the electrogram and thus prevents a possible faulty interpretation of a "normal" electrogram as ventricular fibrillations, based on the detection of these signal components.
In an implantable cardioverter/pacemaker according to DE 37 39 014 A1, the AGC can be used further in connection with the detection of signal components with low amplitude and thus a ventricular fibrillation.
Such a function of the AGC is illustrated in FIG. 3, where the drawn-out graph represents an electrogram where a sinus rhythm can be seen in the left segment (range A), a ventricular tachycardia in the center segment (range B) and ventricular fibrillations in the right segment (range C). The upper, dashed line represents the effective detection threshold as it is adjusted by the AGC, and the events demonstrated for the illustrated, time-dependent course of the detection threshold are shown in the lower section of the Figure.
Certain appearances of ventricular fibrillation are characterized in the intracardial electrogram by the occurrence of a relatively low-frequency signal pattern with comparably high amplitude, superimposed on the fibrillation signals with considerably higher frequency and weaker fibrillation signals; compare in this case perhaps U.S. Pat. No. 4,523,595, especially FIGS. E12 and E13. Such an electrogram is shown (diagrammatically) in FIG. 4, for which the layout is analogous to FIG. 3. As is illustrated with this Figure, with the relatively slow rise of the amplification and the correspondingly slow decline of the detection threshold after each signal of the superimposed signal pattern that respectively increases the threshold, the AGC prevents a detection of the signals that characterize the fibrillation. As a result of this, the defibrillator cannot become operational, even though it may be needed.
With an automatic defibrillator, the automatic gain control (AGC) therefore can possibly result in grave operational defects--not to mention the fact that its realization for strongly differentiated signal images in the form of cardiac electrograms is not simple and is rather costly.